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AN1042 Datasheet(PDF) 1 Page - ON Semiconductor |
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AN1042 Datasheet(HTML) 1 Page - ON Semiconductor |
1 / 12 page © Semiconductor Components Industries, LLC, 2002 August, 2002 – Rev. 3 1 Publication Order Number: AN1042/D AN1042/D High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs Prepared by: Donald E. Pauly ON Semiconductor Special Consultant Almost all switching amplifiers operate by generating a high frequency square wave of variable duty cycle. This square wave can be generated much more efficiently than an analog waveform. By varying the duty cycle from 0 to 100%, a net dc component is created that ranges between the negative and positive supply voltages. A low pass filter delivers this dc component to the speaker. The square wave must be generated at a frequency well above the range of hearing in order to be able to cover the full audio spectrum from dc to 20 kHz. Figure 1 shows a square wave generating a sine wave of one–ninth its frequency as its duty cycle is varied. 0.5 1.0 0 ° 90 ° 180 ° 270 ° 360 ° 420 ° 0.75 0.25 –0.5 0 –0.25 –0.75 –1.0 Input Output Switching Frequency = 9X Modulation Frequency 0 +1 –1 Figure 1. Switching Amplifier Basic Waveforms The concept of switching amplifiers has been around for about 50 years but they were impractical before the advent of complementary TMOS power MOSFETs. Vacuum tubes were fast enough but they were rather poor switches. A totem pole circuit with supply voltages of ±250 volts would drop about 50 volts when switching a current of 200 milliamps. The efficiency of a tube switching amp could therefore not exceed 80%. The transformer needed to match the high plate impedance to the low impedance speaker filter was impractical as well. With the introduction of complementary bipolar power transistors in the late 1960s, switching amplifiers became theoretically practical. At low frequencies, bipolar transistors have switching efficiencies of 99% and will directly drive a low impedance speaker filter. The requirement for switching frequencies above 100 kHz resulted in excessive losses however. Bipolar drive circuitry was also complex because of its large base current requirement. With the advent of complementary (voltage/current ratings) TMOS power MOSFETs, gate drive circuitry has been simplified. These MOS devices are very efficient as switches and they can operate at higher frequencies. A block diagram of the amplifier is shown in Figure 2. An output switch connects either +44 or –44 volts to the input of the low pass filter. This switch operates at a carrier frequency of 120 kHz. Its duty cycle can vary from 5% to 95% which allows the speaker voltage to reach 90% of either the positive or negative supplies. The filter has a response in the audio frequency range that is as flat as possible, with high attenuation of the carrier frequency and its harmonics. A 0.05 ohm current sense resistor (R27) is used in the ground return of the filter and speaker to provide short circuit protection. The negative feedback loop is closed before the filter to prevent instabilities. Feedback cannot be taken from the speaker because of the phase shift of the output filter, which varies from 0 ° at dc to nearly 360° at 120 kHz. Since the filter is linear, feedback may be taken from the filter input, which has no phase shift. Unfortunately, this point is a high frequency square wave which must be integrated to determine its average voltage. The input is mixed with the square wave output by resistors R4 and R5 shown in Figure 2. The resultant signal is integrated, which accurately simulates the effect of the output filter. The output of the integrator will be zero only if the filter input is an accurate inverted reproduction of the amplifier input. If the output is higher or lower than desired, the integrator will generate a negative or positive error voltage. This error voltage is applied to the input of the switch controller, which makes the required correction. The integrator introduces a 90 ° phase shift at high frequencies which leaves a phase margin of nearly 90 °. APPLICATION NOTE http://onsemi.com This document may contain references to devices which are no longer offered. Please contact your ON Semiconductor represen- tative for information on possible replacement devices. |
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